Method for manufacturing soi wafer and soi wafer

ABSTRACT

The present invention provides a method of producing an SOI wafer, comprising at least steps of forming an oxygen ion-implanted layer by implanting oxygen ions into a silicon wafer from one main surface thereof, subjecting the silicon wafer to oxide film-forming heat treatment to convert the oxygen ion-implanted layer into a buried oxide film, and thereby producing an SOI wafer having an SOI layer on the buried oxide film, wherein when the buried oxide film is formed in the silicon wafer, the buried oxide film is formed so that a thickness thereof is thicker than a thickness of the buried oxide film which the SOI wafer to be produced has, and thereafter the silicon wafer in which the thicker buried oxide film is formed is subjected to a heat treatment to reduce the thickness of the buried oxide film. Thereby, there can be provided a method of producing an SOI wafer in which a high quality SOI wafer having a buried oxide film of which a film thickness is thin and perfectness is high and an SOI layer of which crystallinity and surface quality are extremely good can be produced by using SIMOX method.

TECHNICAL FIELD

The present invention relates to a method of producing an SOI (Siliconon insulator) wafer having SOI structure in which a silicon layer isformed on an insulator, and an SOI wafer produced by the method thereof.

BACKGROUND ART

Recently, an SOI wafer having SOI structure in which a silicon layer (anSOI layer) is formed on an insulator has been especially attractingattention as a wafer for high-performance LSI for an electronic devicebecause the SOI wafer is excellent in high-speed property, low powerconsumption, high breakdown voltage, environmental resistance, etc. ofthe device.

Representative production methods of the SOI wafer are bonding method,SIMOX (Separation by ion-implanted oxygen) method, and so on. Thebonding method is a method of producing an SOI wafer by forming an oxidefilm on at least one of a bond wafer to form an SOI layer and a basewafer to be a supporting substrate, bonding the bond wafer to the basewafer through the oxide film, and making the bond wafer into a thinfilm.

Also, the SIMOX method is a method of producing an SOI wafer (SIMOXwafer) by subjecting to heat treatment to form an oxide film in asilicon wafer after implanting oxygen ions into the silicon wafer. Moreconcretely, as shown in FIG. 2 for example, first a silicon wafer 11subjected to mirror polishing etc. is prepared (Step (a′)), andsequentially an oxygen ion-implanted layer 12 is formed by implantingoxygen ions (O+) into the silicon wafer 11 heated at a temperature ofabout 500° C. from one main surface thereof at Step (b′). In this case,as an ion implantation condition, implantation energy is set generally150-200 keV, and dose amount of oxygen ions is required to be high doseamount, about 1.5×10¹⁸/cm² or more, to form a continuous buried oxidefilm.

And thereafter, at a Step (c′), oxide film-forming heat treatment toconvert the oxygen ion-implanted layer 12 formed in the wafer into aburied oxide film 13 is performed, for example, in an atmosphere of aninert gas at a temperature of 1300° C. or more. Thereby SOI wafer 15 inwhich an SOI layer 14 is formed on the buried oxide film 13 can beproduced.

The SOI wafer produced by SIMOX method as described above has advantagesthat there can be easily obtained excellent film thickness uniformitybecause a film thickness of the SOI layer and the buried oxide film canbe determined by the ion implantation energy and dose amount whenperforming the oxygen ion implantation. And also the SOI wafer can beproduced from one silicon wafer without requiring two wafers as theabove bonding method. Therefore the SOI wafer can be produced atcomparatively low cost.

However, in the case of performing the oxygen ion implantation by thehigh dose amount as above by using SIMOX method, although perfectness ofthe buried oxide film can be enhanced, there is an problem that it isdifficult to obtain an SOI layer having a good crystal quality becausedamage is introduced to the wafer surface transmitted by a large amountof the oxygen ions so that threading dislocation is easily generated athigh density in the SOI layer when performing the oxide film-formingheat treatment.

To solve such problems, various studies and investigations have beenrepeated. Through them, it was found that a continuous buried oxide filmcould be formed by oxygen ion implantation even at low dose amount.Thereby, an SOI wafer of which density of threading dislocation is lowcame to be produced (See “Science of SOI”, edited by UCS SemiconductorSubstrate Technology Workshop, published by Realize publishers, pp.26-30). In this case, because the dose amount of the oxygen ions islimited to about 4×10¹⁷/cm², the range of the dose amount is known as adose window.

Though threading dislocation in the buried oxide film formed by such lowdose amount can be reduced, pin holes causing insulation failure of theburied oxide film are easily formed. Therefore a quality of the buriedoxide film is lower than that of the buried oxide film formed by highdose amount.

Accordingly, various methods are suggested to improve the quality of theburied oxide film formed by ion implantation by such low dose amount.For example a method was suggested that the quality of the buried oxidefilm is improved by so-called Internal Thermal Oxidation treatment (itis also described, “ITOX treatment” hereafter) in which after generaloxide film-forming heat treatment, an oxidation treatment in the oxygenatmosphere at high temperature is sequentially performed (see,Publication of Japanese Patent No. 3036619). Thus, by adding ITOXtreatment to SIMOX method performing oxygen ion implantation by such lowdose amount, the buried oxide film is thickened so that the qualitythereof is improved. Therefore it has become possible to produce a SIMOXwafer with high quality in which density of pin holes is low andperfectness of the buried oxide film is enhanced.

However, with higher integration of semiconductor devices in theseyears, production of a higher-quality SOI wafer is desired and, forexample, an SOI wafer having a buried oxide film with a thin thicknessis required. Moreover, it is believed that, in future, a thickness ofthe buried oxide film of an SOI wafer would become even thinner from 100nm to 50 nm or less.

However, when an SOI wafer is produced by using SIMOX method as above,for example if oxygen ions are implanted by high dose amount required toform the continuous buried oxide film, a thickness of a buried oxidefilm formed an SOI wafer always become thicker than one thickness, andit was impossible to form the buried oxide film with a thin thickness asdescribed above.

Also, when an SOI wafer is produced by implanting oxygen ions by lowdose amount, about 4×10¹⁷/cm² (dose window), the buried oxide film withthin thickness can be formed. However, because a quality of the buriedoxide film is low, it is necessary to perform ITOX treatment to improvethe quality of the buried oxide film as described above, and the buriedoxide film in the silicon wafer is thickened to be thick one.Accordingly, it was extremely difficult to produce, by SIMOX method, anSOI wafer having the buried oxide film of which perfectness is high andthe film thickness is thin as required in these days.

Moreover, an SOI wafer produced by the above SIMOX method is excellentin film thickness uniformity and production cost, compared to an SOIwafer produced by the aforementioned bonding method. However, there areproblems that crystal defects existing in the SOI layer are much andcrystallinity of the SOI layer is inferior and also surface roughness ofthe SOI layer is large. Therefore, in production of an SOI wafer bySIMOX method, improvement of crystallinity of an SOI layer andimprovement of surface quality are desired.

DISCLOSURE OF THE INVENTION

Accordingly, the present invention was conceived in view of the aboveproblems. An object of the present invention is to provide a method ofproducing an SOI wafer in which by using SIMOX method, there can beproduced a high quality SOI wafer having a buried oxide film of whichthe film thickness is thin and perfectness is high, and an SOI layer ofwhich crystallinity and surface quality are extremely good.

In order to accomplish the above object, according to the presentinvention, there is provided a method of producing an SOI wafer,comprising at least steps of forming an oxygen ion-implanted layer byimplanting oxygen ions into a silicon wafer from one main surfacethereof, subjecting the silicon wafer to oxide film-forming heattreatment to convert the oxygen ion-implanted layer into a buried oxidefilm, and thereby producing an SOI wafer having an SOI layer on theburied oxide film, wherein when the buried oxide film is formed in thesilicon wafer, the buried oxide film is formed so that a thicknessthereof is thicker than a thickness of the buried oxide film which theSOI wafer to be produced has, and thereafter the silicon wafer in whichthe thicker buried oxide film is formed is subjected to a heat treatmentto reduce the thickness of the buried oxide film.

As described above, in a method of producing an SOI wafer by using SIMOXmethod, when the buried oxide film is formed in a silicon wafer, theburied oxide film is once formed so that a thickness thereof is thickerthan a thickness of the buried oxide film desired in the SOI wafer to beproduced, and thereafter the silicon wafer in which the thicker buriedoxide film is formed is subjected to a heat treatment to reduce thethickness of the buried oxide film. Thereby there can be easily produceda high quality SOI wafer having a buried oxide film of which filmthickness is thin, for example, 100 nm or less and further less than 50nm and perfectness is enhanced. Also, because the thickness of theburied oxide film is reduced by the heat treatment as described above,the part of the reduced thickness is reduced to be a silicon layer withgood crystallinity. Moreover, because an SOI layer is grown bysolid-phase growth from the silicon layer with good crystallinity as aseed in the heat treatment to reduce the thickness of the buried oxidefilm, crystallinity of the SOI layer becomes extremely good, andconcurrently surface roughness of the SOI layer is improved so as toimprove its surface quality.

In this case, it is preferable that the heat treatment to reduce thethickness of the buried oxide film is performed in an atmosphere of ahydrogen gas, an argon gas, or a mixed gas of those at a temperature of1000° C. or more.

By performing the heat treatment to reduce the thickness of the buriedoxide film under the condition, the thickness of the buried oxide filmcan be effectively reduced so that the buried oxide film having adesired thin thickness can be obtained certainly, and also crystallinityand surface quality of the SOI layer can be certainly improved.

Also, it is preferable that oxygen concentration contained in the heattreatment atmosphere of the heat treatment to reduce the thickness ofthe buried oxide film is 10 ppm or less.

When the heat treatment to reduce the thickness of the buried oxide filmis performed, if an oxygen is contained at concentration of more than 10ppm in the heat treatment atmosphere, it is feared that the surface ofthe SOI wafer in the heat treatment is etched so that surface roughnessand film thickness uniformity of the SOI layer become worse.Accordingly, oxygen concentration contained in the heat treatmentatmosphere of the heat treatment to reduce the thickness of the buriedoxide film is thus 10 ppm or less, and thereby the wafer surface isprevented from etching in the heat treatment, and there can be producedan SOI wafer formed an SOI layer having excellent surface roughness andfilm thickness uniformity.

Moreover, it is preferable that material of a wafer boat and/or a heattreatment tube used in the heat treatment to reduce the thickness of theburied oxide film is Si, SiC, or at least one coated thereby on theinternal wall surface.

As described above, if a wafer boat and/or a heat treatment tube ofwhich material is Si, SiC, containing no oxygen as a main component, orat least one coated thereby on the internal wall surface is used, oxygenconcentration in the heat treatment atmosphere can be maintained low.And therefore the etching caused in the wafer surface in the heattreatment to reduce the thickness of the buried oxide film as describedabove can be certainly prevented.

Also, in the method of producing an SOI wafer of the present invention,it is preferable that when the buried oxide film is formed in thesilicon wafer, the silicon wafer is subjected to internal thermaloxidation treatment of the buried oxide film after the oxidefilm-forming heat treatment is performed.

As described above, if the internal thermal oxidation treatment of theburied oxide film, namely ITOX treatment, is added to form a buriedoxide film after the silicon wafer is subjected to the oxidefilm-forming heat treatment, it easily becomes possible to thicken thethickness of the buried oxide film more than the thickness of the buriedoxide film in the SOI wafer to be produced. Moreover, for example, inthe case of forming a buried oxide film by implanting oxygen ions by lowdose amount, the quality of the buried oxide film can be improved byperforming the ITOX treatment. Thus the extremely high quality SOI waferin which perfectness of the buried oxide film is high can be produced.

Also, it is preferable that after the heat treatment to reduce thethickness of the buried oxide film is performed, a sacrificial oxidationtreatment is further performed in order to adjust the film thickness ofthe SOI layer.

As described above, after the heat treatment to reduce the thickness ofthe buried oxide film, by performing a so-called sacrificial oxidationtreatment in which a thermal oxide film is further formed on the SOIlayer and then the oxide film is eliminated, a damage layer generated inthe surface of SOI wafer by oxygen ion implantation can be eliminated,and a crystal quality of the SOI layer is further enhanced as well asthe film thickness of the SOI layer can be adjusted.

And, according to the present invention, there can be provided an SOIwafer produced by the above-described method of producing an SOI waferof the present invention.

If an SOI wafer is produced by the method of producing an SOI wafer ofthe present invention, there can be provided a high quality SIMOX waferhaving a buried oxide film of which the film thickness is thin andperfectness is high, and an SOI layer of which crystallinity and surfacequality is extremely good.

Particularly, in the present invention, it is possible that a thicknessof the buried oxide film of the SOI wafer is less than 50 nm.

As described above, the SOI wafer of the present invention can be anvery high quality SIMOX wafer in which a thickness of the buried oxidefilm is less than 50 nm, which was conventionally difficult to produce.

As explained above, according to the present invention, when using SIMOXmethod, there can be easily produced an high quality SOI wafer in whicha film thickness of a buried oxide film is thin and perfectness is high,and crystallinity and surface quality of an SOI layer are extremelygood.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart illustrating an example of a method of producingan SOI wafer by SIMOX method according to the present invention.

FIG. 2 is a flowchart illustrating a conventional method of producing anSOI wafer by SIMOX method.

FIG. 3 is a graph illustrating a relation between heat treatment time ofthe heat treatment to reduce a thickness of a buried oxide film and areduction amount of a thickness of the buried oxide film, and a relationbetween a thickness of an SOI layer formed in an SOI wafer and areduction amount of a thickness of the buried oxide film.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will now be described.However, the present invention is not limited thereto.

With higher integration of semiconductor devices, it is required that aburied oxide film formed in an SOI wafer is thin such as 100 nm or lessin film thickness, and moreover in future it is believed that therequirement for the thinner buried oxide film will be severe, and thus,production of an SOI wafer having a buried oxide film thickness of, forexample, 50 nm or less, 20 nm, or 10 nm will be required.

However, in the case of producing an SOI wafer by using SIMOX method, afilm thickness of the buried oxide film naturally becomes thick asdescribed above, and therefore, in the production of an SOI wafer byconventional SIMOX method, it was impossible to produce a high qualitySOI wafer having a buried oxide film of which perfectness is high and afilm thickness is thin.

Accordingly, the inventors of the present invention have assiduouslystudied and investigated a method of producing an SOI wafer in whichhaving a buried oxide film of which a film thickness is thin andperfectness is enhanced. As a result, they found that when a buriedoxide film is formed in a silicon wafer, the buried oxide film is onceformed so that a thickness thereof is thicker than a thickness of theburied oxide film which the SOI wafer to be finally produced has, andthereafter the silicon wafer in which the thicker buried oxide film isformed is subjected to a heat treatment to reduce the thickness of theburied oxide film, and thereby a film thickness of the buried oxide filmof the SOI wafer produced by SIMOX method can be reduced, and moreoverthereby there can be produced a high quality SOI wafer having the buriedoxide film of which a film thickness is thinner and perfectness ishigher compared to conventional one, and an SOI layer of whichcrystallinity and surface quality are extremely good. Finally thepresent invention was completed.

Hereinafter, a method of producing an SOI wafer of the present inventionwill be explained in detail by illustrating a case where a buried oxidefilm is formed by implanting oxygen ions by low dose amount (a dosewindow) by SIMOX method with reference to the drawings. However thepresent invention is not limited thereto. Here, FIG. 1 is a flowchartillustrating an example of a method of producing an SOI wafer by SIMOXmethod according to the present invention.

First, a silicon wafer 1 subjected to mirror polishing is prepared (Step(a)). Thus, if a silicon wafer is subjected to mirror polishing,flatness in a mirror polishing surface of the wafer is approximatelymaintained in the SOI wafer to be produced, and therefore there can beobtained an SOI wafer having high flatness.

Sequentially, an oxygen ion-implanted layer 2 is formed by implantingoxygen ions (O⁺) into a silicon wafer 1 heated at a temperature of about500° C. from one main surface thereof to a predetermined depth at Step(b). In this case, a condition of the ion implantation is not limitedparticularly. However, to perform the ion implantation, for example,implantation energy may be set about 150-200 keV used generally andcommonly, and dose amount may be set low dose amount, about4.0×10¹⁷/cm², so as to prevent generation of threading dislocation inthe following oxide film-forming heat treatment. In this case, theoxygen ion implantation may be performed in parts if necessary.

After an oxygen ion-implanted layer 2 is formed in the silicon wafer 1,the oxide film-forming heat treatment to convert the oxygen-ionimplanted layer 2 to a buried oxide film 3 is performed at step (c).Heat treatment condition of the oxide film-forming heat treatment is notparticularly limited, so long as the oxygen-ion implanted layer can beconverted to the buried oxide film. However, for example, the buriedoxide film 3 can be formed by performing heat treatment for 3-6 hours attemperature of 1300° C. to the melt point of silicon in an argon gasatmosphere in which oxygen concentration is 1% or less. In this case, inthe buried oxide film 3, the film thickness is thin and pin holes areeasily formed on account of low dose amount of oxygen ions.

Next, the Internal Thermal Oxidation treatment (ITOX treatment) of theburied oxide film is subjected to the silicon wafer to improve thequality of the buried oxide film at step (d). For example, the siliconwafer in which the buried oxide film 3 was formed is subjected to ITOXtreatment for several hours at temperature of 1150° C. to the melt pointof silicon in an atmosphere of an oxygen gas, and thereby a buried oxidefilm 4 which is thickened can be formed in the silicon wafer. Thus byperforming ITOX treatment, pin holes of the buried oxide film can bereduced and also roughness of interface between the SOI layer and theburied oxide film can be improved, and therefore there can be obtained aburied oxide film with excellent quality.

In addition, if a thickness of the buried oxide film 3 immediately afterthe oxide film-forming heat treatment at step (c) is thicker than athickness of the buried oxide film desired for the SOI wafer to befinally produced, the above ITOX treatment is not necessarily performed.However, even in this case, the quality of the buried oxide film can beimproved by adding the ITOX treatment.

Also, if the silicon wafer is thus subjected to ITOX treatment, athermal oxide film 5 is formed on the wafer surface.

And then, after the thermal oxide film 5 formed on the wafer surface isremoved by etching, or mechano-chemical polishing, etc, the siliconwafer formed the buried oxide film 4 of which the film thickness isthick is subjected to heat treatment to reduce the thickness of theburied oxide film at step (e). Thus, by performing the heat treatment toreduce the thickness of the buried oxide film, there can be produced anSOI wafer 8 having a buried oxide film 6 of which the film thickness isreduced to the desired thickness, and an SOI layer 7 on the buried oxidefilm 6. A thickness of the buried oxide film 6 of the SOI wafer 8finally obtained is determined by product specifications, and accordingto the present invention, there can be formed a very thin buried oxidefilm of which thickness is 100 nm or less, further 50 nm or less, andalso less than 50 nm.

In addition, the heat treatment to reduce the thickness of the buriedoxide film is performed after ITOX treatment in the above flowchart.However ITOX treatment may be performed after the heat treatment toreduce the thickness of the buried oxide film, and also they may beperformed repeatedly so as to farther improve the quality of the buriedoxide film.

Also in the present invention, because the heat treatment is thusperformed to reduce the film thickness of the buried oxide film, thepart of the reduced thickness is reduced to be a silicon layer with goodcrystallinity. Moreover because an SOI layer is grown by solid-phasegrowth from the silicon layer with good crystallinity as a seed in theheat treatment, crystallinity of the SOI layer can be enhanced, andconcurrently surface roughness of the SOI layer can be also enhanced toimprove the surface quality thereof.

In addition, when the heat treatment is performed to reduce a thicknessof the buried oxide film, if the thermal oxide film 5 formed by ITOXtreatment remains on the wafer surface, it is difficult to reduce thefilm thickness of the buried oxide film. Accordingly, if an oxide filmis formed on the surface of the silicon wafer, it is preferable that theheat treatment to reduce the thickness of the buried oxide film isperformed without the oxide film on the wafer surface by removing theoxide film on the wafer surface as described above.

Also, the heat treatment condition of the above heat treatment to reducethe thickness of the buried oxide film may be determined according toneed, and is not particularly limited. However, for example, the heattreatment may be performed in an atmosphere of a hydrogen gas, an argongas, or a mixed gas of those at a temperature of 1000° C. or more,preferably 1100° C. or more, and more preferably 1150° C. or more. Theheat treatment to reduce the thickness of the buried oxide film isperformed under such a heat treatment condition so as to reduce thethickness of the buried oxide film effectively. And therefore there canbe easily obtained the buried oxide film having the thickness, forexample, less than 100 nm, such as 10-80 nm, as to be productspecifications.

Here, there will be shown experimental results concerning a relationbetween heat treatment time of the heat treatment to reduce thethickness of the buried oxide film and the reduction amount of thethickness of the buried oxide film, and a relation between the thicknessof the SOI layer formed in the SOI wafer and the reduction amount of thethickness of the buried oxide film.

First, in order to investigate the relation between the heat treatmenttime and the reduction amount of the thickness of the buried oxide film,there were prepared three silicon wafers having an SOI layer with athickness of 276 nm by removing a thermal oxide film formed on the wafersurface after ITOX treatment. Next, each silicon wafer was subjected toheat treatment to reduce the thickness of the buried oxide film in the100% argon gas atmosphere at 1200° C. for heat treatment times of 1hour, 2 hours or 4 hours, and thereafter, reduction amount of thethickness of the buried oxide film under each heat treatment conditionwas measured. The measurement of reduction amount of the thickness ofthe buried oxide film was performed by measuring the thickness of theburied oxide film of each silicon wafer before and after the heattreatment with a Multilayer Optical Spectrometric Scanner (manufacturedby SOPRA).

As a result, as shown in FIG. 3, it was found that as the heat treatmenttime becomes longer, the reduction amount of the thickness of the buriedoxide film becomes larger. Although not shown in FIG. 3, in the case ofthe same heat treatment time, as the heat treatment temperature becamehigher, the reduction amount of the thickness of the buried oxide filmbecame larger, and when the heat treatment temperature was less than1000° C., the reduction amount of the thickness of the buried oxide filmbecame even smaller.

Next, there was prepared a silicon wafer having an SOI layer of which athickness was 160 nm, and the silicon wafer was subjected to heattreatment to reduce the thickness of the buried oxide film in the 100%argon gas atmosphere at 1200° C. for 1 hour. Thereafter, reductionamount of the thickness of the buried oxide film was measured similarlyas described above. The result of the measurement is shown in FIG. 3together.

As clear from FIG. 3, by reducing the thickness of the SOI layer formedon the buried oxide film from 276 nm to 160 nm, the reduction amount ofthe thickness of the buried oxide film in the heat treatment can beincreased. It was revealed that the reduction amount of the thickness ofthe buried oxide film changed according to a thickness of the SOI layer.Accordingly, when the heat treatment to reduce the thickness of theburied oxide film is performed, it is desirable to determine the heattreatment condition by also controlling a thickness of an SOI layerformed on a buried oxide film.

Also, when the above heat treatment to reduce the thickness of theburied oxide film is performed, if oxygen concentration contained in theheat treatment atmosphere is more than 10 ppm, it is feared that in theheat treatment the surface of the SOI wafer is etched so that surfaceroughness and film thickness uniformity of the SOI layer become worse.Accordingly, when the heat treatment to reduce the thickness of theburied oxide film is performed, it is preferable that oxygenconcentration contained in the heat treatment atmosphere is 10 ppm orless. Thereby the surface of the SOI wafer is prevented from etching inthe heat treatment, and there can be produced an SOI wafer formed an SOIlayer having excellent surface roughness and film thickness uniformity.

Also, the heat treatment apparatus used when the above heat treatment toreduce the thickness of the buried oxide film is performed is notparticularly limited, and a generally used heat treatment device can beused. However, if, for example, a quartz boat and a quartz tube whichare generally frequently used is used for a wafer boat holding a waferand for a heat treatment tube being a reaction chamber, it is fearedthat by affect of oxygen contained therein as an main component, thesurface of the SOI wafer in the heat treatment is etched so that surfaceroughness and film thickness uniformity of the SOI layer become worse asdescribed above.

Accordingly, it is preferable that material of the wafer boat and/or theheat treatment tube used in the heat treatment to reduce the thicknessof the buried oxide film is Si, SiC, containing no oxygen as a maincomponent, or at least one coated thereby on the internal wall surface.By using a wafer boat or a heat treatment tube made of such material,oxygen concentration in the heat treatment atmosphere can be maintainedlow so that the etching caused on the wafer surface in the heattreatment can be certainly prevented.

Moreover, in a method of producing an SOI wafer of the presentinvention, it is preferable that after performing the heat treatment toreduce the thickness of the buried oxide film, a so-called sacrificialoxidation treatment by which thermal oxide film is formed on the SOIlayer and then the oxide film is eliminated is performed.

For example, after performing the heat treatment to reduce the thicknessof the buried oxide film, a thermal oxide film is formed on the surfaceof the SOI layer by performing heat treatment in an oxidizingatmosphere, and then the oxide film formed on the surface of the SOIlayer may be removed by etching with a solution containing HF. Ifetching is performed with a solution containing HF as described above,only the oxide film is removed. Thus, there can be obtained an SOI waferin which damage and contamination, such as heavy metals, are eliminatedby the sacrificial oxidation.

As described above, by further performing the sacrificial oxidationtreatment after the heat treatment to reduce the thickness of the buriedoxide film, a damage layer generated by oxygen ion implantation on thesurface of the SOI wafer can be certainly eliminated. And crystalquality of the SOI layer can be further enhanced as well as the filmthickness of the SOI layer can be adjusted, and thereby there can beproduced a higher quality SOI wafer.

By producing an SOI wafer by the method as described above, a thickerburied oxide film is once formed so as to stabilize the quality, andthereafter the thickness of the buried oxide film is reduced byperforming heat treatment to reduce the thickness of the buried oxidefilm. Therefore, there can be produced a high quality SOI wafer in whicha buried oxide film of which the film thickness is thin and perfectnessis high and in which crystallinity and surface quality of an SOI layerare extremely good.

Hereinafter, the present invention will be explained further in detailwith reference to Examples and Comparative Examples. However, thepresent invention is not limited thereto.

EXAMPLE 1

Mirror-polished silicon wafers having a diameter of 200 mm were preparedto produce SOI wafers having a buried oxide film with a thickness of 80nm as product specifications by SIMOX method.

First, an oxygen ion-implanted layer was formed in the silicon wafer byimplanting oxygen ions into the silicon wafer heated at a temperature ofabout 500° C. from one main surface thereof under a condition thatimplantation energy is set 180 keV and dose amount is set 4.0×10¹⁷/cm².Sequentially, after buried oxide film-forming heat treatment isperformed in an argon gas atmosphere in which an oxygen concentration is0.5% at 1350° C. for 4 hours to convert the oxygen ion-implanted layerto a buried oxide film, the buried oxide film in the silicon wafer wasthickened by performing ITOX treatment in a mixed gas atmosphere of anargon gas and an oxygen gas (oxygen concentration is 70%) at 1350° C.for 4 hours.

Next, after the oxide film formed on the wafer surface was etched toremove with a solution containing HF, the obtained silicon wafer wassubjected to heat treatment to reduce the thickness of the buried oxidefilm in an argon gas atmosphere (oxygen concentration was 10 ppm orless) at 1200° C. for 4 hours. Therefore there was produced an SOI waferhaving a buried oxide film of which the thickness was 80 nm by reducingthe thickness of the buried oxide by 30 nm.

COMPARATIVE EXAMPLE 1

Mirror-polished silicon wafers having a diameter of 200 mm wereprepared. First, an oxygen ion-implanted layer was formed by implantingoxygen ions into the silicon wafer heated at a temperature of about 500°C. from one main surface thereof under a condition that implantationenergy was 180 keV and dose amount was 4.0×10¹⁷/cm². Sequentially, afterperforming buried oxide film-forming heat treatment in an argon gasatmosphere in which an oxygen concentration was 0.5% at 1350° C. for 4hours to convert the oxygen ion-implanted layer to a buried oxide film,ITOX treatment was performed in a mixed gas atmosphere of an argon gasand an oxygen gas (oxygen concentration was 70%) at 1350° C. for 4 hoursto thicken the buried oxide film in the silicon wafer. Therefore therewas produced an SOI wafer having a buried oxide film with the thicknessof 110 nm (Heat treatment to reduce the thickness of the buried oxidefilm was not performed).

COMPARATIVE EXAMPLE 2

Mirror-polished silicon wafers having a diameter of 200 mm wereprepared. First an oxygen ion-implanted layer was formed by implantingoxygen ions into the silicon wafer heated at a temperature of about 500°C. from one main surface thereof in condition that implantation energywas 180 keV and dose amount was 4.0×10¹⁷/cm². Sequentially, buried oxidefilm-forming heat treatment was performed in an argon gas atmosphere inwhich oxygen concentration was 0.5% at 1350° C. for 4 hours to convertthe oxygen ion-implanted layer to a buried oxide film. Therefore therewas produced an SOI wafer having a buried oxide film with the thicknessof 80 nm (ITOX treatment and heat treatment to reduce the thickness ofthe buried oxide were not performed).

As to SOI wafers produced in the above-described Example 1 andComparative Examples 1 and 2, after each SOI wafer was immersed to HFsolution, HF defects formed in the SOI layer of each SOI wafer and pinholes formed in the buried oxide film thereof were scanned by an opticalmicroscope to measure densities of them. Results of these measurementsare shown in Table 1 with production conditions of the above SOI wafersas follows. TABLE 1 Comparative Comparative Example 1 Example 1 Example2 Oxygen ion Implantation energy: 180 keV, implantation Dose amount: 4.0× 10¹⁷/cm² Oxide film-forming In an Ar gas atmosphere in which oxygenheat treatment concentration was 0.5% at 1350° C. for 4 hours ITOXtreatment In a mixed gas atmosphere Non of an Ar gas and an oxygen gas(oxygen concentration was 70%) at 1350° C. for 4 hours Buried oxide film110 110 80 thickness Heat treatment to In the 100% Non reduce thicknessof argon gas buried oxide film atmosphere at 1200° C. for 4 hours (※)Final thickness of 80 110 80 buried oxide film Density of HF <1 <1 <1number/cm² defects number/cm² number/cm² Density of pin 15 15 50number/cm² holes number/cm² number/cm²(※) O₂ amount contained in the Ar gas ≦ 10 ppm,Material of heat treatment tube: SiC,Material of wafer boat: Si

As shown in Table 1, it was revealed that the SOI wafer of the presentinvention (Example 1) have the buried oxide film with quality equal tothe conventional SIMOX wafer subjected to ITOX treatment (ComparativeExample 1) although the film thickness of the buried oxide film thereofis thin, 80 nm. On the other hand, the SOI wafer of Comparative Example2 was not subjected to ITOX treatment to form a buried oxide film ofwhich the film thickness is 80 nm, and therefore pin holes wereremarkably generated in the buried oxide film and the quality of theburied oxide film was very low.

And, after SOI wafers were made again under the same condition asExample 1, and Comparative example 1, further a sacrificial oxidationtreatment was performed to produce SOI wafers having an SOI layer ofwhich a thickness was 30 nm or less. And, density of HF defects of eachSOI wafer was measured and compared similarly as described above. As aresult, it was hardly deferent between both of the SOI wafers. Namely,it was confirmed that, as to the SOI wafer of the present invention,there can be obtained sufficient crystallinity in an area where SiO₂ isreduced to be an SOI layer by heat treatment to reduce the thickness ofthe buried oxide film.

EXAMPLE 2

Several SOI wafers made under the same condition as above Example 1 andhaving buried oxide film with a thickness of 80 nm were prepared. Heattreatment to reduce the thickness of the buried oxide film which wasperformed in an argon gas atmosphere (Oxide concentration was 10 ppm orless) at 1200° C. was added to them by adjusting heat treatment time.Thereby there could be obtained SOI wafers of which final buried oxidefilm thickness were 40 nm, 20 nm, 10 nm, and 5 nm. And, as to the SOIwafers, as a result of measurement of density of HF defects and densityof pin holes similarly as Example 1, they were observed to be equal tothose of Example 1 and Comparative example 1.

As described above, according to the present invention, even if SIMOXmethod by which an SOI wafer can be produced from one wafer is adoptedwithout using bonding method which requires two wafers when producing anSOI wafer, there can be obtained a high quality SOI wafer of whichburied oxide film thickness is less than 50 nm, which could not beobtained by conventional SIMOX method.

The present invention is not limited to the embodiments described above.The above-described embodiments were mere examples, and those having thesubstantially same structure as that described in the appended claimsand providing the similar working effects are included in the scope ofthe present invention.

For example, in the above-described embodiments, there is produced anSOI wafer mainly by implanting oxygen ions by low dose amount (dosewindow). However, the present invention is not limited thereto and alsocan be similarly applied to the case of producing an SOI wafer byimplanting oxygen ions by high dose amount. For example, after a buriedoxide film is formed by implanting oxygen ions by high dose amount sothat a thickness thereof is thicker than a desired thickness, a heattreatment to reduce the thickness of the buried oxide film is performed,and thereby there can be produced a desired SIMOX wafer in which aburied oxide film of which film thickness is thin is formed.

1-8. (canceled)
 9. A method of producing an SOI wafer, comprising atleast steps of forming an oxygen ion-implanted layer by implantingoxygen ions into a silicon wafer from one main surface thereof,subjecting the silicon wafer to oxide film-forming heat treatment toconvert the oxygen ion-implanted layer into a buried oxide film, andthereby producing an SOI wafer having an SOI layer on the buried oxidefilm, wherein when the buried oxide film is formed in the silicon wafer,the buried oxide film is formed so that a thickness thereof is thickerthan a thickness of the buried oxide film which the SOI wafer to beproduced has, and thereafter the silicon wafer in which the thickerburied oxide film is formed is subjected to a heat treatment to reducethe thickness of the buried oxide film.
 10. The method of producing anSOI wafer according to claim 9, wherein the heat treatment to reduce thethickness of the buried oxide film is performed in an atmosphere of ahydrogen gas, an argon gas, or a mixed gas of those at a temperature of1000° C. or more.
 11. The method of producing an SOI wafer according toclaim 10, wherein oxygen concentration contained in the heat treatmentatmosphere of the heat treatment to reduce the thickness of the buriedoxide film is 10 ppm or less.
 12. The method of producing an SOI waferaccording to claim 9, wherein material of a wafer boat and/or a heattreatment tube used in the heat treatment to reduce the thickness of theburied oxide film is Si, SiC, or at least one coated thereby on theinternal wall surface.
 13. The method of producing an SOI waferaccording to claim 9, wherein when the buried oxide film is formed inthe silicon wafer, the silicon wafer is subjected to internal thermaloxidation treatment of the buried oxide film after the oxidefilm-forming heat treatment is performed.
 14. The method of producing anSOI wafer according to claim 10, wherein when the buried oxide film isformed in the silicon wafer, the silicon wafer is subjected to internalthermal oxidation treatment of the buried oxide film after the oxidefilm-forming heat treatment is performed.
 15. The method of producing anSOI wafer according to claim 11, wherein when the buried oxide film isformed in the silicon wafer, the silicon wafer is subjected to internalthermal oxidation treatment of the buried oxide film after the oxidefilm-forming heat treatment is performed.
 16. The method of producing anSOI wafer according to claim 12, wherein when the buried oxide film isformed in the silicon wafer, the silicon wafer is subjected to internalthermal oxidation treatment of the buried oxide film after the oxidefilm-forming heat treatment is performed.
 17. The method of producing anSOI wafer according to claim 9, wherein after the heat treatment toreduce the thickness of the buried oxide film is performed, asacrificial oxidation treatment is further performed in order to adjustthe film thickness of the SOI layer.
 18. The method of producing an SOIwafer according to claim 10, wherein after the heat treatment to reducethe thickness of the buried oxide film is performed, a sacrificialoxidation treatment is further performed in order to adjust the filmthickness of the SOI layer.
 19. The method of producing an SOI waferaccording to claim 11, wherein after the heat treatment to reduce thethickness of the buried oxide film is performed, a sacrificial oxidationtreatment is further performed in order to adjust the film thickness ofthe SOI layer.
 20. The method of producing an SOI wafer according toclaim 12, wherein after the heat treatment to reduce the thickness ofthe buried oxide film is performed, a sacrificial oxidation treatment isfurther performed in order to adjust the film thickness of the SOIlayer.
 21. The method of producing an SOI wafer according to claim 13,wherein after the heat treatment to reduce the thickness of the buriedoxide film is performed, a sacrificial oxidation treatment is furtherperformed in order to adjust the film thickness of the SOI layer. 22.The method of producing an SOI wafer according to claim 14, whereinafter the heat treatment to reduce the thickness of the buried oxidefilm is performed, a sacrificial oxidation treatment is furtherperformed in order to adjust the film thickness of the SOI layer. 23.The method of producing an SOI wafer according to claim 15, whereinafter the heat treatment to reduce the thickness of the buried oxidefilm is performed, a sacrificial oxidation treatment is furtherperformed in order to adjust the film thickness of the SOI layer. 24.The method of producing an SOI wafer according to claim 16, whereinafter the heat treatment to reduce the thickness of the buried oxidefilm is performed, a sacrificial oxidation treatment is furtherperformed in order to adjust the film thickness of the SOI layer.
 25. AnSOI wafer produced by the method of producing an SOI wafer according toclaim
 9. 26. The SOI wafer according to claim 25, wherein the thicknessof the buried oxide film of the SOI wafer is less than 50 nm.